JAK kinases are Janus family nonreceptor protein-tyrosine kinases (NR-PTK) that lack transmembrane regions and form functional complexes with the intercellular regions of other cell surface receptors. They were first identified as the products of mutant oncogenes in cancer cells where their activation was no longer subject to normal cellular controls. Described JAK kinases include Jak1, Jak2, and Jak3, which all share the conserved kinase domain. In addition, these proteins have 5 to 100 amino acid residues located on either side of, or inserted into loops of, the carboxyterminal kinase domain which allow the regulation of each kinase as it recognizes and interacts with its target protein. Known target proteins include growth hormone receptor, prolactin receptor, erythropoietin receptor, cytokine receptors and others which utilize the common chain known as gp130. These receptors are unique both in their ability to recruit multiple PTKs and in the diversity of their responses within different cell types (Taniguchi T (1995) Science 268:251–55). Genetic evidence places these kinases in the interferon α and γ signal transduction pathways which are widely expressed in mammalian cells.
Kinases regulate many different cell proliferation, differentiation, and signaling processes by adding phosphate groups to proteins. The high energy phosphate which drives activation is generally transferred from adenosine triphosphate molecules (ATP) to a particular protein by the PTKs, and the transfer process is roughly analogous to turning on a molecular switch. When the switch goes on, the kinase activates a metabolic enzyme, regulatory protein, receptor, cytoskeletal protein, ion channel or pump, transcription factor, or another kinase. For example, in their normal role, the JAK NR-PTKs are capable of regulating tyrosine phosphorylation of STAT proteins, signal transducers and activators of transcription, such that they translocate to the nucleus and bind DNA (David M et al. (1995) Science 269:1721–1723). In contrast, uncontrolled kinase signaling has been implicated in inflammation, oncogenesis, arteriosclerosis, and psoriasis.
Almost all kinases contain a similar 250–300 amino acid catalytic domain. The N-terminal domain generally folds into a two-lobed structure to bind and orient ATP (or GTP) donor molecules. The larger C terminal lobe binds the protein substrate and carries out the transfer of phosphate from ATP to the hydroxyl group of a tyrosine residue. The primary structure of the kinase domain is conserved in the residues: G50 and G52 in subdomain I, K72 in subdomain II, G91 in subdomain III, E208 in subdomain VIII, D220 and G225, in subdomain IX, and the amino acid motifs of subdomain VIB, VIII and IX (Hardie G and Hanks S (1995) Academic Press, San Diego Calif.).
The novel human Jak2 kinase, hjak2, of the present application shows significant conservation of the diagnostic kinase residues which allowed its identification from among the isolated cDNAs of a placenta library, the anatomy and physiology of which is briefly described below.
The placenta is a thickened disk-shaped temporary organ that interchanges gases, nutrients, hormones, excretory products, humoral antibodies (IgG), and any other circulating substances between the maternal and fetal bloodstreams. Receptors facilitate the transport of glucose, amino acids, and IgG directly from maternal blood to fetal blood. The placenta is the only organ composed of cells derived from two individuals, the fetal extraembryonic chorion and the maternal endometrium. The boundary between these two tissues is marked by extracellular products of necrosis referred to as fibrinoid. This boundary results from the various tissue interactions, immunological responses, etc. which occur in the placenta.
The major tissue interaction involves the expression of paternal antigens by the chorionic villi which is directly adjacent to maternal blood. Although the mother initiates an immunological response, fetal tissue is not typically rejected. This is attributed to the fact that the fetus only expresses major histocompatibility complex (MHC) I, and not MHC II which is the major cause of organ allograft rejection. In addition, uterine secretions during early gestation contain significant amounts of glucose and glycoproteins which may participate in local immunosuppression. Although infections by bacteria, viruses, mycoplasmas, or parasites may ascend from the endocervical canal or reach the placenta through maternal blood, they rarely cause gross pathological changes because of maternal immune defense.
Soon after implantation, fetal villi begin to control maternal physiology to create an optimal environment for development. This involves the production of chorionic gonadotropin, estrogen and progesterone, chorionic somatomammotropin, insulin-like growth factors, platelet derived growth factor, prolactin, and various cytokines. These and other factors such as hjak2 certainly regulate the numerous activities (respiratory, immunological, gastrointestinal, and urinary) which occur within the placenta and between maternal and fetal tissues.
The anatomy and physiology of human placenta is reviewed, inter alia, in Benirschke and Kaufmann, (1992) Pathology of the Human Placenta, Springer-Verlag, New York N.Y., pp. 542–635; Herrera Gonzalez and Dresser (1993) Dev Comp Immunol 17(1):1–18; Mitchell et al. (1993) Placenta 14:249–275; Naeye (1992) Disorders of the Placenta, Fetus, and Neonate; Diagnosis and Clinical Significance, Moseby Year Book, St. Louis Mo.; and Rutanen (1993) Ann Med 25:343–347.